1/ Clearance between the relay contacts and everything else. I don't know what voltages are being switched, but it wouldn't want to be 230VAC.

2/ Input protection for the 7805. I can see a reverse polarity diode and a capacitor but that's all. I'd slap a transzorb and a polyswitch on the input to take care of transients.

3/ For the opto inputs - all I can see is pullup resistors on the port pins. A bit of R & C would go a long way to cope with input transients.


Depending on what the relays are switching, some snubbing across the load or the contacts will help enormously.

The closeness of the DC input to the relay contact wiring is a bit of a worry, make sure this is separated. Make sure the input wiring is separated from the switched loads to minimise pickup.

You then need to look carefully how the DC circuits are wired up. If everything is run from a common power supply, be sure to star wire the power distribution.


The rS485 doesn't look like it has any protection. My usual protection is sa5.0 transzorbs and 100mA polyswitches in series along with 10R resistors on the + and - signals.

From a quick look, it doesn't look like your board has anything that the average PLC doesn't have as standard. Small PLCs are quite cheap these days so I'd need a compelling reason to design something that could be done with such a PLC.